This description relates to patterning semiconductor layers using phase shifting and assist features.
Modern microelectronic devices are commonly produced using a lithographic process. In this process, a semiconductor wafer is coated with a layer of resist. This resist layer is then exposed to illuminating light by passing the light through a mask. The mask controls the amplitude of the light incident upon the wafer. The mask layer is subsequently developed, non-exposed resist is removed, and the exposed resist produces an image of the mask on the wafer.
Different masks are used for patterning the various layers of semiconductor devices. Some layers, such as the layers containing metal and transistors, require masks capable of patterning features with high aspect ratios, i.e., length to width ratios of greater than 2.5. These features are small in only one dimension. Other layers, such as contact and via layers, require masks operable to pattern features with low aspect ratios, i.e., length to width ratios of less than 2.5. These features are small in both dimensions.
Continued improvements in lithography have enabled the printing of increasingly finer features, allowing for smaller device dimensions and higher density devices. This has allowed the integrated circuit (IC) industry to produce more powerful and cost-effective semiconductor devices. As features, which are all smaller than the wavelength of the light used to transfer the pattern onto the wafer, become increasingly smaller, it becomes increasingly more difficult to accurately transfer the pattern onto the wafer.
To solve this problem, two separate techniques have been applied in the development of masks used to pattern layers with high aspect features—phase shifting and the use of assist features. Phase-shifted masks are masks that not only block light, but also selectively alter the phase of the light transmitted through the mask in order to improve the resolution of the features on the wafer. Under subwavelength conditions with closely spaced features, optical distortions as well as diffusion and loading effects of photosensitive resist and etch processes cause printed line edges to vary. By phase shifting the light incident on adjacent features such that certain open regions in the mask transmit substantially all the radiation incident thereon, and near or surrounding open regions transfer all of the radiation incident thereon, at a phase shift of approximately 180 degrees, the spillover of light between one feature and the next causes destructive interference providing a good contrast at the feature's edge. Using phase shifting, nested features can be moved more closely together and can still be accurately patterned.
Assist features, by contrast, are used to pattern isolated high aspect features. Assist features are reticle or mask features used to nest these otherwise isolated features in order to take advantage of photoresist and tools which are optimized to pattern nested features. Assist features are ideally designed such that they are small enough that they do not themselves transfer onto the wafer, but are large enough such that proximal features assume properties of nested features. Assist features also have the advantage of increasing the uniformity of the wafer by ensuring that all features are patterned as nested features.